Requires CONFIG_LOG_BUF_MAGIC; otherwise no error checking will be
performed.

A log record is valid if:
1) The magic value matches
2) the length recorded in the header matches the sum of the lengths of the
subelements.

If a log record fails this check, perform a linear search
to find the next matching log record.

Change-Id: I584350c8bc1bf93c2d956fa1e05db8207cc2d40d